Problem To determine the levels of peripheral blood natural killer (NK) cells in healthy women and recurrent aborters, and the effect of intravenous immunoglobulins (IVIGs) on these levels. Method of study A total of 659 women were evaluated for NK cells by means of flow cytofluorimetry: 42 non-pregnant healthy women, 394 non-pregnant recurrent spontaneous abortion (RSA) women, 36 pregnant healthy women and 187 pregnant RSA women. Fifty-four of the pregnant RSA women were treated with IVIG; in 18 of them NK cells were measured immediately before and after the very first IVIG infusion (0.5 g/kg body weight). Results Blood NK cell results were increased in RSA pregnant/non-pregnant women, and significantly reduced by IVIG, even after the very first infusion. In RSA pregnant women treated by means of IVIG therapy, 92.3% success rate was observed. Conclusions High levels of NK cells are detected in women affected by RSA. IVIGs are capable of decreasing them with a short- and long-term efficacy, allowing having a very high success rate of pregnancies in RSA women.